A hydrostatic transmission is equipped with a variable capacity swash-plate hydraulic pump and a hydraulic motor. When a swash-plate hydraulic pump is driven by an engine, the pump produces high-pressure operating oil. When this high-pressure operating oil is supplied to the hydraulic motor, the hydraulic motor rotates. The drive wheels can be turned by the hydraulic motor.
The engine operates continuously at a constant speed. The rotational speed of the input shaft of the swash-plate hydraulic pump is constant, but the amount of operating oil ejected can be changed by varying the angle of inclination of the swash-plate from the exterior. The hydraulic motor can be rotated normally, stopped, and reversed by varying the angle of inclination of the swash plate. The rotational speed of the output shaft of the hydraulic motor can be varied by adjusting the angle of inclination of the swash plate while the motor is rotated normally or reversed.
Using a hydrostatic transmission makes it possible to maintain a preferred rotational speed in the engine, to control the speed of the vehicle, to move the vehicle forward, to stop the vehicle, or to move the vehicle in reverse.
If an engine is designed to drive left and right hydrostatic transmissions, wherein a left drive wheel is driven by the left hydrostatic transmission and a right drive wheel is driven by the right hydrostatic transmission, a difference in speed can be created between the left and right drive wheels simply by creating a difference in the swash-plate angles of inclination between the left and right hydrostatic transmissions. As a result, the vehicle can be arbitrarily turned to the left or to the right without steering the vehicle.
Japanese Patent Laid-Open Publication No. 7-309149 (JP 7-309149 A), for example, discloses a hydraulically driven vehicle wherein left and right hydrostatic transmissions are driven by one engine. This hydraulically driven vehicle is described with reference to FIGS. 11 through 13 hereof.
As shown in FIG. 11, an engine 102 is mounted in a vehicle frame 101, a left hydrostatic transmission 103L is connected to the engine 102, a left drive wheel 104L is driven by the left hydrostatic transmission 103L, a right hydrostatic transmission 103R is also connected to the engine 102, and a right drive wheel 104R is driven by the right hydrostatic transmission 103R.
Next, the gear-shifting operation will be described.
When a shift lever 105 disposed in proximity to the right drive wheel 104R is operated, a left push-pull rod 106L and a right push-pull rod 106R move together to the left or right of the drawing. Shift arms 107L, 107R protruding from the hydrostatic transmissions 103L, 103R are thereupon swung simultaneously, and the rotational speeds and rotational directions of the left and right drive wheels 104L, 104R are controlled. Specifically, merely operating the shift lever 105 allows the vehicle to be varied between moving forwards and backwards, and the vehicle speed to be varied among stopping, low speeds, and high speeds.
Next, the turning operation will be described.
When a steering wheel 111 is turned, either a left wire 112L or a right wire 112R is pulled taut, while the other is slackened. These actions are converted by a box-shaped transmission 113 to create a difference between the amount of movement in the left push-pull rod 106L and the amount of movement in the right push-pull rod 106R. As a result, a difference in speed is created between the left drive wheel 104L and the right drive wheel 104R, and the vehicle turns to the right or the left.
The detailed structure of the transmission 113 will now be described with reference to FIGS. 12 and 13.
A rotating shaft 115 extending towards the front and back of the drawing spans across a base member 114 that is lowered from the vehicle frame 101, as shown in FIG. 12. The rotating shaft 115 is mechanically connected to the shift lever 105 via a sub-lever 116, a rod 117, and a sub-lever 118 extending from the shift lever 105. Therefore, the rotating shaft 115 is rotated by the movement of the shift lever 105. A pipe 119 extends upward from the rotating shaft 115, and a mixing lever 122 is provided at the top of the pipe 119 via a pin 121.
A gate-shaped guide member 125 is attached to a pair of pivoting shafts 124, 124 extending from the base member 114, with the guide member capable of swinging to the front and back of the drawing. A large groove 126 is formed in the top surface of the guide member 125, and the top of the mixing lever 122 is inserted into this groove 126. The groove 126 is shaped along an arc centered on the rotating shaft 115, and the mixing lever 122 does not come out of the groove even when the mixing lever 122 swings along with the rotation of the rotating shaft 115.
The guide member 125 swings around the pivoting shafts 124 and comes into proximity of either the left push-pull rod 106L or the right push-pull rod 106R, as shown in FIG. 13. A detailed description is not given here, but when the guide member 125 comes into proximity of the left push-pull rod 106L or the right push-pull rod 106R, a difference is created between the amount of movement in the left push-pull rod 106L and the amount of movement in the right push-pull rod 106R.
The transmission 113 described above has a complicated and bulky structure due to being composed of the pivoting shafts 124, 124, the guide member 125, the rotating shaft 115, the mixing lever 122, and other components. Mounting a complicated and large transmission 113 in a small vehicle makes the vehicle more expensive and larger.
In view of this, there is a demand to make the transmission smaller and simpler.